1. Field
The present invention relates to methods and systems for determining vessel compliance and using the determined compliance to calculate blood flow. The invention also includes methods and systems for calculating pulmonary vascular resistance and systemic vascular resistance from pressure versus time measurements for a particular compliance, without directly measuring the pulmonary capillary wedge pressure.
2. State of the Art
Cardiac output, which is the rate of flow (in liters per minute) of blood from the heart, is one measure of cardiac hemodynamics. Various procedures have been developed to determine cardiac output. The standard procedure for measuring cardiac output involves using a thermal dilution catheter, such as a Swan-Ganz flow-directed thermal dilution catheter.
In a common procedure, a thermal dilution catheter, such as a Swan-Ganz catheter, is inserted into a jugular vein in the neck. The catheter is connected to tubes through which air and liquids may be injected. To place the catheter, a balloon on the tip of the catheter is inflated. The blood flow to the heart pulls the catheter through the right ventricle into the pulmonary artery. A thermistor near the tip of the catheter is used in measuring the temperature of the blood. To determine the cardiac output, the clinician injects about 3 cubic centimeters (cc) of ice cold saline through one of the tubes to an opening some distance from the tip of the catheter. The saline mixes with and cools the blood in the right atrium of the heart. As the heart pumps, the cooled blood and saline eventually are pumped past the thermistor. The temperature of the thermistor is measured and a temperature versus time profile is generated from which the cardiac output may be calculated by a well known technique, such as the classical Stewart Hamilton equation modified for use with a thermal indicator. More specifically, the area under the temperature versus time curve is proportional to the cardiac output.
There are several sources of error in the results of thermal dilution procedures. Sources of error include variations in the rate at which the cold saline is injected, the temperature of the saline, and the position of the catheter and thermistor within the vessel. The calculated cardiac output may be .+-.20% or more than the actual cardiac output. Typically, the thermal dilution procedure is employed three times and an average is made of the three calculated cardiac outputs. However, multiple applications of the procedure is unsatisfactory because the patient is subjected to multiple injections of cooled saline and the attendant risk of problems, such as infection. Also, the average of three calculated cardiac outputs is still not particularly accurate.
A more recent variation of the thermal dilution procedure is described in U.S. Pat. No. 4,507,974 ("'974 patent") to Yelderman. The system described in the '974 patent includes an intravascular heating element that heats the blood, as opposed to cooling the blood through injecting a cold saline. The temperature of the blood is measured through a thermistor. The rate at which the measured temperature changes is used in calculating cardiac output. Disadvantages of the system of the '974 patent include potential blood damage as a result of using a heating element and that the accuracy of the calculated cardiac output has error approximately the same as standard thermal dilution.
Another technique for determining cardiac output was based on the work of Homer Warner in the early 1950's, which is described in A Guyton et al., "Circulatory Physiology: Cardiac Output and its Regulation," (2nd Ed. 1973), chap. 5 (hereinafter "Guyton"). Guyton explains that the during the cardiac output (CO) may be derived from equation (1), below: EQU CO=C.times.F(Pcd-Pab) (1+Sa/Da) (1).
In equation (1), C is the compliance of the vessel (called "capacitance" by Guyton). The compliance is the change in volume of a vessel (in this case the arterial tree) for each unit change in pressure. F is the heart rate. Pcd is the average pressure in the arterial tree at the end of systole. Pab is the average pressure in the arterial tree at the end of diastole. Sa is the area under the arterial pressure curve during systolic drainage (shown in FIG. 5-3 of Guyton). Da is the area under the arterial pressure curve during diastolic drainage (shown in FIG. 5-3 of Guyton).
Solving for C in equation (1) produces equation (2) below: EQU C=CO/(F(Pcd-Pab) (1+Sa/Da)) (2).
Warner assumed that the compliance of a vessel is constant. With that assumption, he reasoned that the cardiac output could be calculated repeatedly through the following procedure, which involves only one flow measurement procedure. The cardiac output (CO) was initially determined using the Fick or similar procedure. The pressures of equation (1) are measured at the same time. The compliance C is determined from the measured cardiac output and pressures. Thereafter, the cardiac output may be calculated merely by determining the heart rate and measuring the pressures of equation (1). The Warner procedure may be applied continuously without injection of an indicator. It is, therefore, much less invasive than repeated Fick or thermal dilution procedures.
A major problem with the Warner procedure is that the compliance is not constant. Compliance changes significantly as, for example, the patient takes vasoactive drugs or as the condition of the patient changes. Accordingly, the Warner procedure may inaccurately calculate the cardiac output rates over time.
A second measure of cardiac hemodynamics is pulmonary vascular resistance ("PVR"), which is the resistance to blood traveling through the lungs to be oxygenated. PVR may be calculated by equation (3), below: EQU PVR=(PAP.times.PCW)/CO (3),
where PAP is the mean pulmonary artery pressure, PCW is the pulmonary capillary wedge pressure, and CO is the cardiac output.
The standard technique for calculating pulmonary vascular resistance is as follows. The wedge pressure is measured by inserting a catheter with a balloon at its end into one branch of the pulmonary artery connected to a lung and inflating the balloon until the artery is completely sealed off. The pressure across the lung is then measured through a lumen at the tip of the catheter. The pulmonary artery pressure is measured through standard techniques such as through pressure transducers connected to a Swan-Ganz catheter. The cardiac output is measured through a thermal dilution catheter as described above. Disadvantages of this procedure for calculating pulmonary vascular pressure include the invasiveness of measuring wedge pressure and all of the above-recited disadvantages of the thermal dilution catheter procedure.
A third measure of cardiac hemodynamics is systemic vascular resistance ("SVR"), which is a measure of the resistance of the blood flow through the body other than the lungs. The systemic vascular resistance gives a measure of the performance of the left heart and the vascular condition of the body. SVR may be calculated by equation (4), below: EQU SVR=(AOP.times.PCW)/CO (4) ,
where AOP is mean arterial pressure, PCW is the pulmonary capillary wedge pressure, and CO is the cardiac output.
A standard technique for systemic vascular resistance is as follows. First, wedge pressure of the pulmonary system is measured. Second, the cardiac output is calculated. Third, the mean arterial pressure is measured.